In a semiconductor manufacturing process, there may be a case where a film forming process of forming a silicon nitride (SiN) film on a substrate is performed so as to cover an underlying film on which a pattern such as a hole, a groove or the like is formed. Conventionally, a film forming apparatus for forming an SiN film in this way has been known. According to this film forming apparatus, the SiN film is formed on the substrate by allowing the substrate loaded on a rotary table to alternately repeatedly pass through a region to which a raw material gas containing silicon is supplied and a region to which a nitriding gas for nitriding the raw material gas is converted into plasma and supplied by rotation of the rotary table.
Meanwhile, for example, a carbon film using carbon as a main component, which is referred to as an SOC film, is often used as the underlying film. However, in a case where the underlying film is the carbon film as described above, when the processing is performed by using an ammonia (NH3) gas as the nitriding gas and converting the NH3 gas into plasma, the carbon film may be relatively greatly etched, leading to film loss (reduction in film thickness). In order to prevent such etching, it is considered that the processing is performed using a nitrogen (N2) gas converted into plasma, instead of the NH3 gas converted into plasma. However, since active species obtained by converting the N2 gas into plasma are relatively quickly deactivated, there is a possibility that most of the active species does not reach a lower side of hole and groove of the carbon film. In this case, in the SiN film formed on a sidewall of the carbon film on which a pattern is formed, a film thickness of the lower side of the sidewall may be smaller than a film thickness of an upper side of the sidewall. Thus, it was difficult to obtain desirable step coverage in forming the SiN film on the carbon film on which a pattern is formed.